The invention relates to hydrogen storage alloys for use in rechargeable batteries.
A battery typically includes one or more galvanic cells (i.e., cells that produce a direct current of electricity) in a finished package. In each cell, two electrodes are separated by an electron insulator, but are joined by an ion-carrying path. The electron-carrying path of the battery is external; the path proceeds, via a conductor, through a device where work is done. The ion-carrying path of the battery is internal and proceeds via an electrolyte.
The electrodes are usually composed of dissimilar metals. For discharge of a cell, the electrode where the electrolyte is broken down upon the receipt of electrons is the positive electrode, also referred to as the cathode. The electrode where the metal goes into solution, releasing electrons, is called the negative electrode, or anode. The electrolyte generally is composed mainly of an ionizable salt dissolved in a solvent.
Batteries may be rechargeable; such batteries are called "storage" or "secondary" batteries. Storage batteries can be recharged by passing current through the cells in the opposite direction of current flow on discharge. The chemical conditions of the battery are restored, and the cells are ready to be discharged again. Primary batteries, on the other hand, are meant to be discharged to exhaustion once, and then discarded.
An example of a rechargeable battery is a metallic oxide-hydrogen storage battery. The positive electrode of this battery includes a metal oxide, such as nickel oxide; the negative electrode includes a hydrogen storage alloy; and the electrolyte includes an alkaline solution.
An example of an electrode reaction in a nickel oxide-hydrogen storage battery is as follows. Postive electrode: ##STR1##
Native electrode: ##STR2##
In the reaction equation (2), M represents a hydrogen storage alloy. Hydrogen storage alloys are capable of electrochemically absorbing and discharging large quantities of hydrogen. One type of hydrogen storage alloy is the AB.sub.5 -type, which has a crystal structure of the CaCu.sub.5 type. The A and B components of the AB.sub.5 -type alloy are present in a mole ratio of about 1:5. The A component is generally composed of a mischmetal (a mixture of rare earth elements, generally cerium (Ce), lanthanum (La), neodymium (Nd), and praseodymium (Pr), as well as zirconium (Zr)), and the B component is generally composed of nickel (Ni), along with two or more elements selected from cobalt (co), manganese (Mn), aluminum (Al), copper (Cu), iron (Fe), or germanium (Ge). The subscripts, which indicate mole fraction, of the elements forming the A component generally have a sum of 1, while the subscripts of the elements forming the B component generally have a sum of 4.75 to 5.50.
It is desirable for metallic oxide-hydrogen storage batteries to have characteristics such as high energy density, relatively high charge retentions, relatively long cycle lives, and good discharge capacities over a range of temperatures. The hydrogen discharge reaction at the negative electrode, however, tends to slow down with decreasing temperature; discharge capacities may therefore deteriorate at low temperatures. The low temperature performance of batteries can be improved, but improved low temperature performance is often accompanied by the loss of other desirable properties such as high temperature performance, capacity (the ability to reversibly store hydrogen) or cycle life.